The present invention relates to an elastic fiber produced from a composition comprising at least one hydrogenated block copolymer. In particular, the invention pertains to elastic articles such as, for example, but not limited to, elastic fabric, as well as composites comprising the same, especially a composite absorbent item comprising at least one elastic article.
Materials with excellent stretchability and elasticity are needed to manufacture a variety of disposal and durable articles such as, for example, incontinence pads, disposable diapers, training pants, sports apparel and furniture upholstery.
Disposable articles are typically elastic composite materials prepared from a combination of polymer film, fibers, sheets and absorbent materials as well as a combination of fabrication technologies. Whereas the fibers are prepared by well known processes such as spun bonding, melt blowing, melt spinning and continuous filament wounding techniques, the film and sheet forming processes typically involve known extrusion and coextrusion techniques, e.g., blown film, cast film, profile extrusion, injection molding, extrusion coating, and extrusion sheeting.
A material is typically characterized as elastic where it has a high percent elastic recovery (i.e., a low percent permanent set) after application of a biasing force. Ideally, elastic materials are characterized by a combination of three important properties, i.e., a low percent permanent set, a low stress or load at strain, and a low percent stress or load relaxation. That is, there should be (1) a low stress or load requirement to stretch the material, (2) no or low relaxing of the stress or unloading once the material is stretched, and (3) complete or high recovery to original dimensions after the stretching, biasing or straining is discontinued.
Lycra (a spandex supplied by Dupont Chemical Company) is a segmented polyurethane elastic material that is known to exhibit good elastic properties. But Lycra tends to be extremely cost prohibitive in several applications. Also, Lycra like natural rubbers tends to exhibit poor environmental resistance to ozone, chlorine and high temperature, especially in the presence of moisture.
Block polymers generally are elastomeric materials that exhibit excellent solid-state elastic performance attributes. But unsaturated block polymers such as, for example, styrene-butadiene-styrene triblock polymers, tend to exhibit mediocre thermal stability, especially in the molten state and poor UV stability.
Conversely, known partially hydrogenated (or partially saturated) styrene block copolymers (for example, KRATON G block copolymers supplied by Shell Chemical Company) are difficult to melt process and draw into fibers or films. In fact, preparation of fine denier fiber (that is, less than or equal to 40 denier) or thin film (that is, less than or equal to 2 mils) from partially hydrogenated or partially saturated block polymers is generally not possible at commercial fabrication rates. To overcome characteristic melt processing and drawing difficulties, partially hydrogenated block copolymers are commonly formulated with various additives such as oils, waxes and tackifiers. But in order to achieve good melt processability and drawability, very high levels of low molecular weight additives are typically required which tend to compromise strength and elastic properties.
WO 95/33006 discloses blends of styrene block polymers with substantially linear ethylene polymers. This disclosure describes as one advantage of blending with substantially linear ethylene polymers an improvement in processability. That is, the ethylene polymers are described as fusion promoters and processing aids which reduce the processing delay times characteristic of (partially) saturated styrene block copolymers.
Hydrogenated block copolymers of vinyl aromatic and conjugated dienes such as styrene-butadiene-styrene polymers are well known in the art. U.S. Pat. Nos. 3,333,024; 3,431,323; 3,598,886; 5,352,744; 3,644,588 (the disclosures of all of which are incorporated herein by reference) and EP-505,110, disclose various hydrogenated block copolymers. In particular, full hydrogenation of the aromatic ring of the block polymers has been investigated. But polymer scientists contend that fully hydrogenated styrene-butadiene-styrene copolymers (that is, complete saturation of the vinyl aromatic monomer unit as well as the conjugated diene monomer unit) have no useful properties at elevated temperatures, even if only slightly elevated. For example, Thermoplastic Elastomers, 2d edition, 1996, page 304, lines 8-12, states xe2x80x9cThus, polystyrene remains the choice for any amorphous hydrocarbon block copolymer. This last fact is clearly demonstrated in the case of the fully hydrogenated VCH-EB-VCH polymer. The interaction parameter is so severely reduced by hydrogenation that at only slightly elevated temperatures, the polymer loses all strength and appears to be homogeneously mixed at ordinary melt temperatures.xe2x80x9d
In spite of various disclosures relating to elastic materials, including disclosures pertaining to hydrogenated block copolymers as well as blends consisting of block polymers and ethylene polymers, such as for example U.S. Pat. No. 5,093,422 to Himes and SIR No. H1,808 to Djiauw et al., there is a present need for cost-effective elastic materials (and articles thereof) having good processability while maintaining strength and elastic properties.
We have discovered that a composition comprising at least one substantially hydrogenated block copolymer, surprisingly exhibits improved melt drawability and processability, (and in certain embodiments improved elastic properties) while providing retained or improved strength properties in fibers produced therefrom.
We have also discovered that this composition can be conveniently used to prepare improved disposable and durable elastic articles with or without the use of various additives such as processing aids, oils, waxes, polyolefins and tackifiers.
One aspect of the present invention is a fiber produced from a composition comprising at least one substantially hydrogenated block copolymer characterized as having
i) a weight ratio of conjugated diene monomer unit to vinyl aromatic monomer unit before hydrogenation of greater than or equal to 60:40;
ii) a weight average molecular weight (Mw) before hydrogenation of from about 30,000 to about 150,000, wherein each vinyl aromatic monomer unit (A) has a weight average molecular weight, Mwa, of from about 5,000 to about 45,000 and each conjugated diene monomer unit (B) has a weight average molecular weight, Mwb, of from about 12,000 to about 110,000; and
iii) a hydrogenation level such that each vinyl aromatic monomer unit block is hydrogenated to a level of greater than 90 percent and each conjugated diene monomer unit block is hydrogenated to a level of greater than 95 percent, as determined using UV-VIS spectrophotometry and proton NMR analysis.
In one preferred embodiment, the composition, or fiber or article therefrom, is irradiated or crosslinked using any suitable technique, including ultraviolet irradiation and silane curing. Preferably, however, irradiation or crosslinking is effectuated using ionizing radiation provided by electron beam irradiation. Preferably, the extrudate, filament, web or part is permitted to cool or is quenched to ambient temperature (i.e., permitted to substantially solidify) before the application of additional heating or ionizing radiation to effectuate irradiation or crosslinking. Most preferably, the electron beam radiation is conducted under an inert atmosphere such as, for example, under nitrogen. In another embodiment of the invention, the fiber may be produced from a composition additionally comprising at least one other polymeric material such as a homogeneously branched ethylene polymer, especially a substantially linear ethylene polymer.
Surprisingly, it as been discovered that substantially hydrogenated block copolymers, even at molecular weights substantially higher than comparative partially hydrogenated block copolymers, can be successfully melt drawn, including meltspun into fine denier fibers, where the comparative block polymer cannot be melt drawn nor meltspun into fibers at any denier. This discovery is believed to be attributable to the surprising low shear melt viscosities of substantially hydrogenated block copolymers. Ordinarily, polymeric materials with higher molecular weights are expected to exhibit commensurately higher melt viscosities (and subsequently, poor processability and melt drawability) and certainly are not expected to exhibit dramatically lower viscosities.